The Promise of Solar Power
An array of mirrors is focused on a spot on a 200-foot (61-m) “power tower.” It is capable of producing heat of more than 1,000 suns, reaching temperatures of 4200 degrees F (2300 degrees C)
IN AN ERA of energy shortages, it has not gone unnoticed that the sun is an unfailing source of energy, showering its beneficent light and warmth over all the inhabited earth. It maintains the earth at a comfortable average temperature. It furnishes the energy for plant growth, and thus for all life. These benefits are so obvious that many take them for granted.
But we have come to rely on other forms of energy for many uses for which the sun’s radiation is not directly useful. If other sources of energy dwindle and fail, would it be possible to heat our homes and factories with sunbeams? Could we transform the sun’s rays in some way to provide electricity for our lights, to run our motors, and for our radios and television sets? Could we bottle up the sun’s energy in tanks to fuel our automobiles and airplanes?
These possibilities are now being seriously considered. Scientists in many laboratories are doing basic research on ways to utilize the sun’s energy. There is no doubt that the potential is there. The sun’s radiation falling on an area only 16 miles (26 km) square in Arizona carries energy equal to that generated by all the electric power plants in the U.S. What, then, are the problems?
The first problem we face is that sunlight is inherently diffuse. Any collector of limited size receives relatively little energy. But even this diffuse power is sufficient for some uses. Buildings planned to admit sunlight can capture enough heat to save much of the fuel needed for heating. Water can be heated in roof tanks hot enough for bathing, for washing dishes, or for the laundry.
Another limitation inherent in solar power is that it is not always there when we want it. It is turned off at sunset. Clouds, too, shut off the sun’s power. The intensity of sunlight, the number of daylight hours and the amount of cloudy weather all vary with the latitude and the seasons. For many uses, acceptance of solar power will depend on finding ways to store up energy while the sun shines and to use it at night or on cloudy days.
One simple way to store solar energy is to heat water during the day and keep it in insulated tanks for use at night. The hot water can also be circulated through radiators to heat the house. During bad weather, such a system would have to be supplemented from another source. But as an auxiliary heating system, it is already being put to use to reduce the need for gas or electricity.
Going beyond this elementary application are more sophisticated ways to use the sun’s heat. By concentrating the sun’s rays, it is possible to reach much higher temperatures. Who has not tried the experiment of putting a piece of paper under a magnifying glass at the focus of the sun’s rays and watching it smolder and burst into flames? This principle is applied on a large scale, using curved mirrors, to concentrate the sun’s rays to a dazzling white heat on a small area, hot enough to melt the most refractory materials. In such a solar furnace in southern France, a boiler mounted at the focal point is used to generate electricity supplied to the national power system. The manufacturer offers to sell solar power plants with a 1,000-kilowatt capacity.
A more elaborate system of this kind has been built near Albuquerque, New Mexico, to study its economic potential for full-size power plants. An array of mirrors is focused on a spot on a 200-foot (61-m) “power tower.” Each mirror is four feet (1.2 m) square, and 25 of them are mounted in a square pattern on a “heliostat.” As the sun moves across the sky, the heliostat must be tilted in synchronism with the sun’s motion to keep its reflected beam on the target. There are 222 such heliostats set in a triangular field north of the tower. A computer guides each one separately, according to its distance and direction.
When they are focused together on the tower, all the sunlight falling on two acres (0.8 ha) is concentrated on an area of about five square feet (0.5 m2). The heat of more than a thousand suns reaches a temperature of 2,300 degrees C (4,200 degrees F). In early tests, the heliostat beams quickly melted a hole through a steel plate.
After tests with a water boiler in the tower, it is planned to build a 10,000-kilowatt solar power station at Barstow, California, where it can be tied into the power grid in southern California, perhaps as early as 1981.
Electricity from Sunlight
Meanwhile, other scientists are working toward the longer-range goal of converting sunlight directly into electricity. The principle itself is not new. We have been using devices based on the photoelectric effect for years. For example, a photocell in a camera tells the correct lens opening to use for the brightness of the scene before it. The light generates a tiny electric current, which moves a needle on a dial. To scale this up to enough current to do useful work is a formidable undertaking, but one that offers great rewards.
How can light generate electricity in a photocell? The secret lies in the use of a semiconducting element. An element that is a good conductor, such as most metals are, has its electrons very loosely attached to the atoms. They move about freely to carry current. In insulators, the electrons are tightly bound in their orbits, and are not free to move. Semiconductors are in between; the electrons are bound, but not tightly, so that just a little push will free them and let them move about.
Pure silicon is a poor conductor. However, slight amounts of impurities make it a much better conductor. For example, a trace of an element like arsenic, which has five outer electrons, one more than silicon’s four, supplies free electrons to the crystal. Or a little boron, which has only three outer electrons, causes a deficiency. The missing electrons are called holes. Another electron can easily jump into a hole from an adjacent atom, giving the same effect as if the hole were moving, and a positive current flowing.
The first kind of impure silicon is called n-doped silicon, because it has excess electrons (negative). The second kind is called p-doped, because it has excess holes (positive). If these two kinds of silicon are put face to face they form an n-p junction. Electrons will flow in only one direction across this junction. This is the basis of the transistor, which has replaced yesterday’s bulky vacuum tubes with today’s tiny silicon chips.
Now suppose we take two sheets, one each of n and p silicon, and put them together. Instead of the transistor chip, we now have a solar voltaic cell. If this is exposed to the sun, the energy in the photons, the individual packets of sunlight, is absorbed and serves to set electrons free from the silicon atoms. If the two sides of the cell are connected to form a circuit, electrons will flow from the n side to the p. This electric current can be put to work. It is electricity made from sunlight.
Not all the energy in the sunlight can be recovered as electricity. The energy in a photon of sunlight varies from 1.5 to 3.0 electron-volts, as the color ranges from red to violet. But it takes only about 1.0 electron-volt to free the electron in the silicon crystal, so the rest of the energy is lost as heat. The maximum theoretical efficiency of a single silicon cell is about 22 percent. The most efficient cells actually made so far are about 15 percent efficient. It is hoped that, by combining different elemental types of semiconductors in several layers, as much as 50 percent conversion of the energy in sunlight can be achieved.
Applications of Solar Cells
Solar electric cells have already found an important niche in modern technology, being used to supply power to space vehicles. They are ideally suited to this application. In interplanetary travel they are exposed all the time to full sunlight (in orbit, more than half the time). Clouds do not get in the way, and they are not battered by rain or wind. Their cost is absorbed in budgets for space research.
So we find that the most striking feature in the silhouette of the Skylab or the Vikings that went to Mars is the large solar vanes extended from them. The solar power cells have proved reliable and durable. The power plant in the Viking orbiter was still producing 600 watts two years after it arrived at Mars. Its performance in this demanding task certainly recommends it. The meticulous care and extravagant cost of manufacturing solar cells to guarantee such perfection can well be lavished on a Viking. But their present cost will have to be reduced to less than a 20th to make them economically attractive for electric power on earth. This might appear to put the prospect of solar electric power far in the future, but the tremendous cost reductions that we have seen in other semiconductor devices offer hope for earlier success. Workers in many laboratories are actively pursuing research toward automatic processes to make solar cells cheaper. Enthusiastic supporters claim that the sun could be supplying 20 percent of the energy needed in the U.S. by the year 2000.
Solar electric power has one feature that stands in sharp contrast to many other ways of producing electricity. It is inherently modular. That is, the basic unit of production is a single small module. To get more power, one merely joins more modules together. This is not true of steam-generated electricity. It takes a large plant to make power cheaply by burning oil or coal. This is also true with nuclear power, and it will be overwhelmingly true of fusion power. But sun-generated electricity promises to be just as cheap from small plants as from large ones.
This opens a provocative question: Might it be possible to do away with the extensive power networks that are essential in the present system? Perhaps the power plant of the future will be more of a community or neighborhood project, or even adapted to isolated individual dwellings. This thought is disturbing to those who have organized the production of electricity around huge regional, even national networks. It is understandable that industrial leaders who sense a threat to their vast investment in the present system might not be enthusiastic in support of such a radical innovation. If these were not dragging their feet, some claim, solar power could be developed more rapidly.
Other advantages of direct solar electricity are clearly attractive. It will be clean, noiseless and reliable. There are no moving parts and there is nothing to wear out. It is simple to use. It causes no pollution. Its power supply is free and as renewable as sunlight from one day to the next. Do you wonder that the promise of such an energy source stirs advocates to demand every effort to be directed toward its early fulfillment?
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The sunshine falling on 16 square miles in Arizona equals the energy generated by all the electric power plants in the U.S.
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Enthusiastic supporters claim that the sun could supply 20 percent of the commercial energy needed in the U.S. by the year 2000
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Advantages of direct solar electricity: no pollution, no noise, nothing to wear out, and the power supply as free and as renewable as sunlight from one day to the next
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Solar Power from Space
The most incredible idea of all to tap sunshine for electric power is one that might come out of a science-fiction movie. A huge array of solar panels, as much as 50 km2 (20 square miles) total in area, would be assembled out in space. This energy-collecting station would be put in orbit 36,000 km (22,300 miles) high, where it would be stationary over a selected point on the equator. The power generated would be beamed by microwaves to a receiving antenna on the ground, 10 km (6 miles) in diameter. The five million kilowatts produced would be about enough for New York city. This proposal offers one clear advantage over earthbound solar collectors. The space power plant would operate 24 hours a day, and cloudy weather would not interfere with either the collection of energy or its transmission by microwaves.
But such a gargantuan construction does not lie within the scope of present space-age technology. To develop the rockets and to transport the materials and workmen into space would cost many billions of dollars. And one wonders whether the stray microwaves would be a hazard to people near the receiving station. Also, what effect might it have on the ionosphere and the weather, on radio and television? Astronomers complain that these bright objects in the sky would permanently stop their exploration of deep space, because for this they need a dark sky. Utility executives might favor this scheme, because you would still have to depend on their distribution system.
But if you could store energy overnight, you might prefer to take your solar power direct from the sun as it shines on your house, avoiding this elaborately contrived detour. After all, by the time solar satellites become a reality, you may be able to collect enough sun power for your household use with as little as 30 square feet (3 m2) of solar cells on your roof.